1. Field of the Invention:
The present invention relates to a method for epitaxially growing a ZnS(Se) type II-VI compound semiconductor, which is used for a blue light emitting element. More particularly, the present invention relates to a method for epitaxially growing a ZnS(Se) type II-VI compound semiconductor in which the strain and interface defects caused by a lattice mismatch with a GaAs substrate are controlled, and to a semiconductor laser element produced by the method.
2. Description of the Related Art:
A semiconductor laser will be described as an example of a conventional blue light emitting element referring to FIG. 10.
An n-type GaAs substrate 1 is loaded in a molecular beam epitaxial (hereinafter referred to as the "MBE") growth apparatus with a high vacuum. In the apparatus, the surface of the substrate 1 is heated to approximately 600.degree. C. and irradiated with As molecular beams. The surface of the substrate 1 is thus cleaned. Then, an n-type GaAs layer 2 is grown on the substrate 1 to approximately 100 nm at the same temperature by the MBE method. An n-type ZnSe layer 3 is grown on the n-type GaAs layer 2, and then, an n-type ZnSSe layer 4, n-type ZnSe layer 5, a CdZnSe layer 6, a p-type ZnSe layer 7, a p-type ZnSSe layer 8 and a p-type ZnSe layer 9 are successively grown thereon. An element having a sectional structure as shown in FIG. 10 is thus produced (M. A. Haase, J. Qiu, J. M. DePuydt and H. Cheng, Appl. Phys. Lett. Vol. 59 1991, p. 1272). On the p-type ZnSe layer 9, an insulating film 10 is formed. A gold layer 11 and an indium layer 12 are formed as a p-type and an n-type electrodes, respectively, so as to sandwich the thus obtained laminated structure. These layers can be grown in the same or different MBE growth chambers.
The thus produced semiconductor laser can be oscillated by pulse driving at room temperature.
In the element having the above-mentioned laminated structure, the lattice mismatch between the crystal of ZnSe and that of GaAs is about 0.25%. However, the lattice mismatch between the crystal of CdZnSe or ZnSSe, that is, a mixed crystal of Zn together with Cd or together with S, and the crystal of ZnSe is significantly large. Moreover, since a coefficient of thermal expansion of the substrate between the epitaxial growth temperature and room temperature is different from that of each growth layer, a large stress is caused between the substrate and the growth layers. Dislocation is caused by such a stress in each of the growth layers, resulting in extremely shortening the life time of the semiconductor laser. Further, a difference in the band gap on the interface between GaAs and ZnSe (hereinafter referred to as the "hetero interface") is approximately 1.3 eV at room temperature, and largely prevents current injection. The hetero interface also has a problem of impurity diffusion caused by mutual diffusion between the constitutive atoms. Such an impurity can also prevent the current injection.
In this manner, in an element produced by epitaxially growing a ZnSe layer, that is, a II-VI compound layer, on a GaAs substrate, a density of dislocation is extremely high and the element has a very short life time. This is because:
a) Dislocation is caused due to a large lattice mismatch between the substrate and the ZnSe layer; and
b) Dislocation is caused due to a large difference between the substrate and the ZnSe layer in the coefficient of thermal expansion from the growth temperature to room temperature.
In order to solve this problem, an MEE (migration enhanced epitaxy) (Y. Horikoshi et al., Japan J. Appl. Phys. Vol. 25, 1986, L868.) and an ALE (atomic layer epitaxy) have been used to attempt to grow layers at a low temperature. But they are not perfect methods.
In this manner, dislocation is caused and a density of the dislocation can not be reduced in such a conventional semiconductor laser. Moreover, it is considered that 50% or more of the injected current is thermally lost because of a difference in the band gap on the hetero interface of GaAs and ZnSe and a resistance component caused by mutual diffusion on the interface.